- T-Carrier Technologies -

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1 1 T-Carrier Fundamentals - T-Carrier Technologies - T-Carrier systems provide digitized communication for voice or data traffic across a telephone provider s network. The T-Carrier specification defines the Layer-1 aspects of the multiplexed communication. The two most prevalent T-Carrier systems are T1 and T3 circuits. The Digital Signal (DS) refers to the signaling rate and framing of the T- Carrier. The basic unit is the DS0, referred to as a channel or timeslot. The DS0 channel was designed to support one voice call, with a throughput of 64 kbps. It is possible to multiplex multiple DS0 s to form a higher-capacity link: A T1 circuit consists of 24 DS0 channels, for a total throughput of Mbps. A T3 circuit consists of 672 DS0 channels, or 28 T1 circuits, for a total throughput of Mbps. It is also possible to utilize only a subset of channels on a T1, referred to as a fractional T1 (or frac T1). The terms T1 and DS-1 are often (incorrectly) used interchangeably to refer to a 24-channel multiplexed line. Remember: the term T1 refers to the hardware aspect of the technology, whereas DS1 refers to the framing. A T1 line can operate over as few as two copper pairs (4 wires). One pair is used to transmit data, and the other pair is used to receive data. In European and Asian countries, an E-carrier system is used instead of T- carrier. An E1 consists of 30 channels of 64 Kbps, for a total throughput of Mbps. This guide will concentrate solely on the theory and configuration of T1 circuits. T3 and E1 circuits go beyond the scope of this guide. (Reference: CCNP BCRAN Exam Certification Guide, Second Edition. Pages Cisco Press ISBN: )

2 2 T-Carrier Sampling and Framing Recall that T-Carriers provide purely digitized communication, for both voice and data. It was originally developed for voice communication, however. A voice call requires a 4 khz (4000 Hz) channel. To convert analog voice into a digital format, samples of the frequency and amplitude of the analog wave must be made. Sampling, in this instance, refers to representing a snapshot of the signal as a sequence of binary digits (usually 8 bits). The Nyquist sampling theorem dictates that analog wave should be sampled at a rate of twice the channel s frequency range: f s = 2(freq. range) Thus, assuming a range of 4000 Hz, this requires a rate of 8000 samples per second. Each sample is represented using 8 bits of data. Each sample is assigned an 8-bit value to represent the amplitude height at the time of sampling. The first bit of the 8-bit value designates whether the wave s height is positive (1) or negative (0). The remaining seven bits allow for a height range of -127 to 127, for a total of 256 representations of the wave s sampled amplitude. The throughput of a DS0 channel can thus be calculated as such: 8000 samples per second x 8 bits per sample = 64,000 bps (or 64 Kbps) A DS-1 utilizes a CSU (Channel Service Unit) to format data into frames. The size of a single frame is exactly 193 bits. Each channel attaches 8 bits of data per frame, and then one framing bit is attached. Thus, during each sampling period: 24 channels x 8 bits of data per channel + 1 framing bit = 193 bits per frame Assuming a sampling rate of 8,000 frames per second, the total throughput of a DS1 will be: 8000 frames per second x 193 bits per frame = Mbps (Reference: Broadband Telecommunications Handbook. Pages Regis J. Bates. McGraw Hill. ISBN: )

3 3 T-Carrier Framing Protocols T-Carrier systems utilize framing protocols to maintain synchronization and for error control. The two most common framing protocols include: D4 SF (SuperFrame) ESF (Extended SuperFrame) D4 SF utilizes the single framing bit in 12 consecutive frames to apply a repeating pattern of The twelve frames are collectively known as a superframe (hence SF). The continuous pattern allows the sending and receiving devices to stay synchronized. ESF, as its name implies, expands the superframe to 24 consecutive frames. In addition to using the framing bits for synchronization, ESF also employs error control and maintenance services. Specifically: 6 bits of each superframe are used for synchronization 6 bits of each superframe are used for CRC error control 12 bits of each superframe are used for maintenance services, such as performance statistics (Reference:

4 4 T-Carrier Line Coding The two most commonly used types of line coding include: AMI (Alternate Mark Inversion) B8ZS (Bipolar 8 Zero Substitution) Binary data is represented on a T1 circuit using a pulse. A pulse represents a binary 1; the absence of a pulse represents a binary 0. If a long set of consecutive zeros are transmitted, devices may lose synchronization due to the lack of pulses. Thus, the ones density rule was implemented, which states that at least 12.5% (in other words, 1 out of 8) of all transmitted bits must be set to 1. Otherwise continuous strings of zeros may cause timing to be lost. If a CSU receives a string of eight zeroes, it will change the last bit to a binary 1 (known as pulse stuffing). One out of every eight bits has essentially been rendered unreliable, reducing the throughput of the channel by 1/8 th (down to 56,000 bps). This above is the basics of how AMI line coding works. AMI is no longer widely implemented, and has been largely replaced with the more advanced B8ZS standard. B8ZS also adheres to the ones density rule, but utilizes a specific pattern ( ) to replace eight consecutive zeros. The receiving CSU recognizes that this pattern represents all zeroes, and will revert the bits to their original value. B8ZS is thus more efficient than AMI, and does not lose 1/8 th of the throughput of a channel. (Reference: Broadband Telecommunications Handbook. Pages Regis J. Bates. McGraw Hill. ISBN: ;

6 6 Troubleshooting T1 Circuits (continued) If a large number of framing errors are occurring, possible causes can include: Too much noise on the link Defective cable Incorrectly configured CSU line clock One of the clocks is configured for internal clocking Ones density problem on the T1 circuit (Reference: Optical Carriers If throughput beyond the speed of T-carriers or E-carriers is necessary, an Optical Carrier (OC) can be used. OC circuits utilize a SONET fiber network to transmit digital signals. The specifics of OC technology go beyond the scope of this guide. However, the following chart details the speeds for common OC configurations (please note: this is not a comprehensive list): OC-1 OC-3 OC-12 OC-24 OC-48 OC Mbps Mbps Mbps Gbps Gbps Gbps (Reference:

FURTHER READING: As a preview for further reading, the following reference has been provided from the pages of the book below: Title: Broadband Telecommunications Handbook Author: Regis J Bud Bates Publisher:

Appendix D T Overview GENERAL T refers to the primary digital telephone carrier system used in North America. T is one line type of the PCM T-carrier hierarchy listed in Table D-. T describes the cabling,

FURTHER READING: As a preview for further reading, the following reference has been provided from the pages of the book below: Title: Broadband Telecommunications Handbook Author: Regis J. Bud Bates Publisher:

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